Add putchar() function to stdlib
Simple 'echo' program for testing tsh

Echo is a useful program for testing shell command parsing.
Use BSD strtok(_r)

Mine doesn't collapse groups of delimiters, and I don't particularly
feel like fixing it.
Permit single-quoted arguments in shell commands

Arguments containing spaces may be put inside single quotes in order to
be passed to the program unmolested.  There is currently no support for
escaped quote characters.
Include <errnodefs.h> in <kernel/common.h>

Error code definitions should be available throughout the kernel.
Use generic list interface for memory lists

Use the Mach queue interface for managing lists of allocated and free
memory blocks.

A new function (queue_replace_entry()) is added to queue.h to replace
list entries.